The availability of optical fiber for relatively loss-free and environmentally stable transmission and distribution have created much interest in optical control of microwave signals for commercial applications such as cable television (CATV) signal distribution, high definition television (HDTV), communications and phased-array radar. Since the MESFET dominates GaAs MMIC technology, exploring its properties for use as an optical detector is important, particularly if the emphasis is on monolithic integration of optical and microwave components. TO efficiently introduce optical signals into the active regions of FET's for photodetection is important. However, optical coupling via the top of the MESFET is poor due to the shadowing of the optical signal by the gate, drain and source metal contacts.
As is known to those skilled in the art, a FET is comprised of a substrate on which an epitaxial layer is grown and parallel, spaced, metal source, gate and drain electrodes that are deposited on the top of the epitaxial layer. In order for light to influence the operation of the FET, it must be introduced into what is known as the active region that is largely in the portion of the epitaxial layer lying between the source and drain electrodes. If light is directed toward the active region through the substrate, it is largely absorbed by the substrate, and if it is directed toward the active region from the other side, it is largely blocked by the gate electrode. In fact, only 5% of the region is reached in the latter case.